The Latest: Turkey says allied Syrian rebels nearing al-Bab

Turkey's president says allied Syrian opposition fighters are fast approaching the Syrian town of al-Bab, a stronghold of the Islamic State group.

President Recep Tayyip Erdogan said in a televised speech Sunday that "our aim is to go to al-Bab and expel them toward the south," adding that the opposition fighters were up to 9 miles (15 kilometers) from the town.

He did not comment on an announcement by Syrian Kurdish-led forces that they had launched an offensive to recapture the Syrian city of Raqqa, the de facto capital of the extremist group's self-styled caliphate.

Both groups of Syrian fighters have been battling IS in recent months, but they have also turned their guns on each other. Turkey views the Syrian Kurdish forces as an extension of the Kurdish insurgency in its southeast.

The Turkey-backed forces have vowed to drive both IS and the Syrian Kurdish forces from the border.

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3:15 p.m.

Syrian activists say a rocket or mortar barrage struck a preschool in an opposition-controlled suburb of the capital, Damascus, killing at least six children.

The activist-run Unified Medical Bureau of Eastern Ghouta reported the death toll and said medical facilities in the suburb of Harista received tens of wounded children and adults following Sunday's shelling.

The locally-run Education Directorate said government forces struck the school during the first recess of the day. It said more than 25 children were wounded in addition to the six killed.

The Britain-based Syrian Observatory for Human Rights, which relies on a network of local activists, also blamed government forces for the strike.

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1 p.m.

U.S.-backed Kurdish-led Syrian forces have announced the start of a campaign to retake the Islamic State's de facto capital of Raqqa from the extremist group.

The announcement by the Syria Democratic Forces was made Sunday at a press conference in Ein Issa, north of Raqqa.

It comes more than two weeks after U.S.-backed Iraqi forces began a campaign to clear IS militants from their stronghold in Mosul, Iraq.

The SDF is dominated by the main Syrian Kurdish fighting force known as the People's Protection Units, or YPG.

The United States considers the Syrian Kurdish fighters as the most effective force against the IS, but Turkey views them as a terror organization and has said it will not accept a role for the Kurds in the liberation of Raqqa.